Ab urbe condita

Usage of the term was more common during the Renaissance, when editors sometimes added AUC to Roman manuscripts they published, giving the false impression that the convention was commonly used in antiquity. In reality, the dominant method of identifying years in Roman times was to name the two consuls who held office that year. In late antiquity, regnal years were also in use, as in Roman Egypt during the Diocletian era after AD 293, and in the Byzantine Empire from AD 537, following a decree by Justinian.

Use

Prior to the Roman state's adoption of the Varronian chronology – created by Titus Pomponius Atticus and Marcus Terentius Varro – there were many different dates posited for when the city was founded. This state of confusion required picking a canonical founding date for one to use an AUC date. The Varronian chronology, constructed from fragmentary sources and demonstrably about four years off of absolute events c. 340 BC, placed the founding of the city on 21 April 753 BC. This date, likely arrived at by mechanical calculation but accepted with a variance of one year by the Augustan-era fasti Capitolini, has become the traditional date.

From the time of Claudius (r. AD 41–54) onward, this calculation superseded other contemporary calculations. Celebrating the anniversary of the city became part of imperial propaganda. Claudius was the first to hold magnificent celebrations in honour of the anniversary of the city, in AD 47, the eight hundredth year from the founding of the city. Hadrian, in AD 121, and Antoninus Pius, in AD 147 and AD 148, held similar celebrations respectively.

In AD 248, Philip the Arab celebrated Rome's first millennium, together with Ludi saeculares for Rome's alleged tenth saeculum. Coins from his reign commemorate the celebrations. A coin by a contender for the imperial throne, Pacatianus, explicitly states "[y]ear one thousand and first", which is an indication that the citizens of the empire had a sense of the beginning of a new era, a Sæculum Novum.